Fortunately Water Bugs although they have some special adaptations have a varied diet which allows them to succeed whatever food is on offer. They mainly fit into three groups of feeders:

Some are highly adapted Predators like Nepa cinerea that will lie in wait and pounce and then hold other invertebrates with it's modified front legs to kill them. Gerridae feel vibrations on the surface with hairs on their mid and hind legs and then use eye sight to pounce and capture almost any animal either water or land based. One observation was that Nepa cinerea let a spider walk over it but when it aligned infront of it, it grabbed it with its front legs and pulled it towards it's Rostrum in a lightening fast manouvre. 

Some are Gatheror-collectors of detritus, plant or algae matter - mainly Corixidae. Micronecta poweri anchors itself to the bottom in flowing water and uses it's front palae gathering,  then sucking in food and sometimes they also take larger particles that can be broke up by pharyngeal teeth before they reach the gut.

Some are Scrapers and Grazers again mainly Corixidae. Hesperocorixa sp. are mainly detritus feeders whereas Corixa punctata has salivary glands that create a a secretion of enzymes that break down vegetation to be be consumed

If we take all of the main groups:

Notonectidae sp are again highly developed predators with a short but powerful rostrum and toxic saliva that can be very painful even to us. They also use wave detection and good sight to attack. Notonecta glauca is primarily an under water feeder and lies motionless at the surface and attacks underwater disturbances. They will eat almost any swimming animal even including, large tadpoles, full grown Dytiscidae larvae and small fish. 

Notonecta maculata is principally a surface feeder using site and because of its barren habitats it has to also rely on drowning land insects.

Pleadidae sp. main food is cladocera.

Nepidae sp. are ambush predators. Ranatra linearis spends much of it's time immobile within vegetation in which it is camoflaged awaiting water fleas and a wide range invertebrates.  Nepa cinerea uses it's powerful modified front legs to tightly hold the prey after it has been ambushed. It eats small fish, larvae of other water bugs, water beetle larvae and adults, mosquita larvae and many other aquatic insects. 

Corixidae sp. are mainly herbivors feeding on bottom algae with the exceptions of Cymatia sp. , Glaenocorisa sp. and Arctocorixa sp. Cymatia coleoptera use vegetation to hide amongst and then rush out to capture fleas, corixid larvae, mayfly's and chironomid larvae. Apparently it kills more food than it needs. Glaenocorisa propinqua is a large eyed nocturnal feeder that swims rapidly and uses long hairs on front legs to strains out planktonic organisms such as rotifers, copepods, water fleas and algae. Arctocorixa carinata also predates midge larvae and pupa.

Corixid sp. have a rostrum that is more fused with the head but still have stylets that allow them to stab and suck prey. They also have a pair of sclerotic teeth in the gut that allow them to break up food particals and this feature differentiates them from other water bugs. This allows them to take in bits of plant or animals and then masticate that internally. Their palae are also modified into a "spoon" shape for sifting detritus, algae and bottom "ooze". 

Sigara dorsalis and Sigara falleni feed on detritus, living animal material, diatoms and algal filaments.  Hesperocorixa linnei, Hesperocorixa sahlbergi, Sigara lateralis and Sigara stagnalis were all detritus feeders.  There is a great variation of diets across species and different habitats. 

Naucoridae sp feed similarly to Notonectidae. Ilyocoris cimicoides is a fierce carnivore that feeds on Water fleas, Gammarus and Asellus as well as insect larvae and Aphelocheirus aestivalis diet is mainly Chironomid and mayfly larvae.  

Velidae sp. are surface feeders. Velia caprai and Velia saulii use ripples emitted by insects to detect prey including a variety of insects including spiders. Large prey can be taken to the land to consume, carried by the tip of the rostrum. Mesovelia furcata  takes small animals including ostracods which are speared through the surface film.  

Salidae sp. are thought to be predators. Saldula Saltatoria spends its time running and jumping over mud in search of prey. When caught it holds it at the tip of the rostrum sometimes it stands on the prey but never uses its front legs.